We obtained mortality rates for unspecified injury and for cause-specific injury using CDC’s Web-based Injury Statistics Query and Reporting System (WISQARS™). WISQARS™ defines “unspecified injuries” using ICD-10 codes: X59 (unintentional); Y09,*U01.9 (homicide); Y35.7 (legal intervention); X84,*U03.9 (suicide); and Y34, Y89.9 (undetermined intent) .
a) Changes in specificity of injury mortality data
The proportion of unspecified injury deaths was calculated as “unspecified injury mortality divided by all-injury mortality*100%”. Based on published studies [1, 8], we defined the quality of data specificity as ‘improved’ when the proportion decreased; when the proportion increased, we defined the quality as ‘worsened’.
Based on the results of preliminary analysis that revealed similarity in proportions of unspecified injury for some age groups (not shown here), we divided ages into four groups: 0–24 years, 25–44 years, 45–64 years, and 65+. Because unspecified injuries from legal intervention and with undetermined intent accounted for <8% of all unspecified injuries during 1999–2010, we targeted unspecified injuries of three intent-specific categories: unintentional injury, homicide, and suicide.
Linear regression was used to examine the significance of changes in the proportion of unspecified injury deaths by intent and age group from 1999 to 2010.
b) Relationship between changes in mortality rates from unspecified injuries and from cause-specific injuries
For the groups that had high proportions of unspecified injuries and experienced significant changes in the proportion of unspecified injuries between 1999 and 2010, we plotted stacked bar charts to demonstrate changes in mortality rates from unspecified injuries and from cause-specific injuries and performed Spearman correlation to measure the relationship between them. Suicide was excluded from Spearman rank correlation analysis because of very low proportions of unspecified suicide during 1999–2010 (<0.7%).
Based on preliminary analysis (not shown here), seven major unintentional injury causes (falls, motor vehicle traffic crashes/MVT, suffocation, fire/burn, natural/environmental disaster, poisoning, and drowning) and four common homicide mechanisms (firearm, cut/pierce, suffocation, and struck by or against) were targeted for Spearman rank correlation analysis.
c) Impact of changes in specificity of data recording
Only intent- and age-specific groups showing significant changes in unspecified injury mortality during 1999–2010 in step a) and significant correlations between unspecified injury mortality and specific injury mortality in step b) were chosen for redistribution of unspecified injury mortality. In total, four groups were redistributed, including unintentional injury, 65+; homicide, 0–24 years; homicide, 45–64 years; and homicide, 65 + .
To quantify the impact of changes in specificity of data recording, we first adjusted the proportion of unspecified injuries at different years to the same level. Of 1999–2010, we chose the year having the lowest proportion of unspecified injuries as reference. We assumed that the proportions of unspecified injuries in other years could be lowered to the reference level if the specificity of injury data recording improved.
For the years not having the lowest proportion, we then redistributed the “excess unspecified injuries” to cause-specific injuries using proportionate method. Early studies indicated that the proportionate method was useful for approximately redistributing injuries with unspecific codes to cause-specific injuries [2, 11, 12]. The proportionate method supposes that the “excess unspecified deaths” have similar cause-specific proportions as injuries with cause-specific codes in the same year. We performed the redistribution of “excess unspecified injuries” by age group and intent.
Finally, we compared the percent change in cause-specific mortality between 1999 and 2010 before and after the redistribution. The percent change in mortality was calculated as “(age-adjusted mortality in 2010 – age-adjusted mortality in 1999)/age-adjusted mortality in 1999*100%”.
The data were from CDC’s Web-based Injury Statistics Query and Reporting System (WISQARS™) that does not cover any private information and could be accessed freely at http://webappa.cdc.gov/sasweb/ncipc/mortrate10_us.html. This study was approved by the IRB of the School of Public Health, Central South University (China) to be exempt for review. Data analysis was performed between March 1, 2014 and May 15, 2014.